Inter symbol interference ("ISI") is a term which is frequently employed to describe the behavior of magnetic recording systems. ISI may be due to one or more of a number of different causes. When magnetic transitions are placed in very close proximity, pulses may become crowded which changes the amplitudes of the pulses, and which shifts the peaks of the pulses. ISI may also be attributable to certain magnetic data transducer characteristics. One such characteristic, "undershoot", is most frequently associated with e.g. thin film heads. ISI sources may be associated with other head designs and structures, such as magneto-resistive read only/inductive write head structures, for example, which exhibit readback non-linearities.
Thin film heads are now frequently employed within high capacity, high performance digital data storage and retrieval channels, such as in PRML digital magnetic data storage devices. One drawback of e.g. thin film heads is that they typically introduce undershoot artifacts into the electrical signal stream during playback of data from a relatively moving magnetic medium, such as a data track of a magnetic disk M. FIG. 1 illustrates a thin film head element TF in which main field (MF) flux lines are mainly focused between inside edges of two poles P1 and P2 which define the flux transition sensing field. However, because the thin film poles P1 and P2 have finite pole widths, they have outer polar fields (designated by a long flux path line OPF in FIG. 1). When a magnetic flux transition passes by an outer edge of a pole, e.g. P1 in FIG. 1, a "pretransition" bump or "undershoot" is induced in the head structure and manifests itself as an undesirable electrical amplitude spike or pulse US in the electrical output current from the head. The undershoot US may occur on one side, or maybe both sides, of each main flux transition recorded on the magnetic medium. The undershoots typically occur at some measurable time before and/or after the head senses the flux reversal within its primary sensing field, depending on several factors, such as pole tip width, and relative velocity between the magnetic medium and the head. Depending on these factors, the undershoot bump may occur somewhere in the range of 10-20 clock cycle intervals ("T cells"). FIG. 2 illustrates an undershoot occurring at -16 T and another one occurring at +16 T relative to a main transition occurring at 0 T, for example. When a high frequency preamble or sync pattern is being read, the undershoots may line up and undesirably add to the high frequency pulses, leading to timing loop and gain loop convergence errors.
In theory, "infinite pole tip models" are frequently discussed. As the pole tip of the thin film head element gets wider, the bump amplitude becomes smaller, and its occurrence is farther away from the main pulse, as the outer polar field becomes longer and longer.
Referring to FIG. 2, when a signal is properly equalized within a partial response, class IV, system, the main pulse results in two adjacent unitary sample values, while adjacent sample values on each side of the main pulse are quantized at zero amplitude. These samples remain zero until the undershoot(s) is encountered. The absolute value of the undershoot may typically have a peak amplitude of about ten percent of the absolute value of the peak amplitude of the main pulse, as shown in FIG. 2.
The problems associated with undershoot are present everywhere within a PRML class IV digital data channel. Importantly, with regard to the present invention, these problems are most severe during a preamble field which is typically recorded as a frequency burst of constant amplitude sine waves. During playback of user data, a scrambler is employed which scrambles the impact of the undershoots, and they add into the recovered sample stream as an extra noise source. However, a periodic pattern is purposefully chosen to provide a preamble, so that during playback of the preamble simultaneous optimization of orthogonal timing and AGC loops may be carried out. Since timing, gain and DC offset loops bear an orthogonal relationship to each other, they may be operating simultaneously and independently of each other in reaching optimization during the preamble field interval.
Thus, there are really two places where undershoot may cause problems in operation of a PRML digital sample data channel. One is during detection of actual user data bits, and another is during timing loop and gain control acquisition while the preamble field passes by the thin film head.
In order to eliminate this interference during detection of actual user data bits, it is practical to apply an undershoot cancellation filter, sometimes referred to as a "pole tip filter". One example of a pole tip filter is provided in FIG. 3. Before discussing FIG. 3, reference is made to copending, commonly assigned U.S. patent application Ser. No. 07/937,064 filed on Aug. 27, 1992 and entitled: "Disk Drive Using PRML Class IV Sampling Data Detection with Digital Adaptive Equalization", now U.S. Pat. No. 5,341,249 the disclosure thereof being incorporated herein by reference. That patent describes an exemplary PRML digital data channel having a second order timing loop and a first order gain loop, and structural elements associated therewith. Structural elements thereof which are essentially unchanged in the present discussion are assigned the same reference numerals in this discussion.
In FIG. 3, a pole tip filter 101 is interposed in a digital signal path between an adaptive finite impulse response ("FIR") filter 48 and a Viterbi detector 40. In this example, the pole tip filter 101 includes a first FIR filter 103, a first digital sample delay 103 having a predetermined length, such as e.g. 16 T, a second digital sample delay 105 having a predetermined length, such as e.g. 16 T, a second FIR filter 109 and a summing junction 111. The summing junction combines weighted values received respectively from the first FIR 103, a common node between the two delays 105 and 107 and from the second FIR filter 109 to provide a sum in which the undershoot attributed to the outer polar field of the thin film data transducer head TF is effectively canceled. One drawback of the pole tip filter 101 depicted in FIG. 3 is that at least a 32 T latency is required before filtering operation begins. This latency, attributable to the two 16 T delays 105 and 107, means that an excessively long preamble field would have to be provided for timing phase lock, or possibly that the PLL becomes unstable or inaccurate in operation. Such a long preamble field would add excessive overhead to a disk drive, particularly one employing embedded servo sectors which necessitate timing resynchronization to user data following each embedded servo sector. For timing and gain loop acquisition, the process must be started and completed much sooner; therefore preliminary samples (taken before the undershoot filter as at e.g. multiplexer 51) are used.
In the case of a 1/4T sine wave preamble pattern, unless eliminated or effectively canceled, the undershoots can add to the periodic sine wave pulses in a systematic and undesirable fashion because of the periodicity of the preamble pattern. The effect of additive undershoots during the repetitive preamble pattern is that timing phase can become skewed during timing PLL sync lock to the preamble pattern. This phase skew of the timing control loop can result in sampling errors in coded transitions read back from the magnetic storage medium, leading to unacceptable bit error rates at the beginning of the user data field following the preamble field. A phase timing error of as much as five percent has been observed in practice, and this error or phase step in the timing loop has resulted in undue bit errors in as many as the first 10-20 user data bytes of the data field. For example, a phase step PS associated with undershoot-induced timing phase error representative of the prior art is superimposed as one graph upon similar graphs illustrating operation of the present invention in FIG. 6 hereof.
In addition to timing loop phase errors, the undershoot-induced ISI can lead to errors in convergence of the gain control loop. When the undershoots add to the 1/4 T sine wave peaks undesirably, the filtered digital samples taken from the output of the FIR filter 48 will erroneously manifest an incorrect magnitude, and this condition will be properly detected by the gain control circuit 64 and will result in reduction of overall channel gain. When the random user data field is reached and the effects of additive ISI are no longer present, the nominal channel gain will be determined to be too low, and an undesirable gain control step (identified by reference symbols GS in FIG. 6) will be present during the e.g. initial 10-20 bytes of user data. The undesired gain step GS can have an undesired additive effect to the undesired timing control loop phase step PS which further degrades data recovery, until both timing and gain control loops have readjusted to randomized user data.
One possible solution to the problem of the unwanted phase step is to inject a counter-step of proper inverse phase magnitude into the timing loop at the transition between preamble and user data, in order to cancel the effect of the undershoot-induced phase step. However, such an approach favors a digital synchronizer which is amenable to this proposed solution. Other forms of data synchronizers, such as analog synchronizers, are less amenable to injection of a phase step, because their electrical characteristics tend to change or drift with time or temperature. Also, precise undershoot and channel filter settings must be known typically on an individual channel basis in advance.
A gain step could also be inserted into the gain control loop 64, but only with added gain loop structural complexity and a priori information about the nature of the gain step of the particular data channel.
Thus, heretofore an unsolved need has remained for an effective method for eliminating undershoot-induced ISI timing phase steps and gain control loop steps and resultant data sampling errors occurring at the beginning of a user data field in a PRML digital sampling magnetic data storage and retrieval channel employing e.g. thin film data transducer heads.